In this application we propose a series of studies, designed to determine the mechanisms by which human contrast sensitivity is regulated. We are specifically interested in determining whether or not the small movements of the eye, which occur during normal fixation, enhance or inhibit sensitivity to spatial contrast. Futhermore, we would like to attempt to determine the physiological mechanisms that mediate this regulatory function of eye movement. The method is to eliminate the effects of eye movements by stabilizing the retinal image. We plan to compare the measures of sensitivity obtained under conditions when the image is motionless on the retina to the results obtained under conditions when the retinal image moves normally. In addition, we plan to study the behavior of the single cells of the cat visual system in response to small motions of the image on the retina.